SPREADING CENTER
VOLCANISM

Spreading
center volcanism occurs at the site of mid-oceanic ridges,
where two plates diverge from one another. As the plates are pulled
apart, hot asthenosphere
rises upward to fill voids of the extended lithosphere.
The rise of this hot mantle provides thermal buoyancy to the ridge
area and this is the reason that they stand as high ridges in
the center of ocean basins. This is demonstrated in the colored
relief images shown here.

Mid-oceanic ridge
of
the south Pacific.

Mid-Atlantic Ridge.Courtesy of NOAA.

As the hot asthenosphere rises
to shallow levels, it decompresses and melts to produce basaltmagmas. These
magmas pond in crustal magma chambers where they are periodically
tapped by vertical fractures that provide conduits for the rapid
rise of magma to the surface. The eruptions produced in this manner
are typically fissure eruptions.
The erupting basalt can generate vast submarine lava fields. Typically,
the lava quenches quickly against the bottom waters to produce
characteristic bulbous shapes called pillow
basalt.

The
high heat content of mid-ocean ridges is evident from the occurrence
of numerous hydrothermal vents. These form from
surface water that seeps downward through cracks where it heated
by hot rocks lying above the magma chambers. These hot thermal
waters then ascend back through the overlying crust, where they
leach out silica and numerous metals from the basaltic lava. The
hot springs created at the surface are called a black
smokers because they are readily identified by billowing
dark clouds composed of metal-rich fluids.

Above:
pillow basalt from the south Pacific.

Right:
black smoker from
the mid-Atlantic Ridge.

As basaltic
lava erupts at the surface, more or less continuously for millions
of years, it is constantly accreted onto the edge of the spreading
plates as it cools into a hardened basalt layer. This process
generates oceanic crust. Oceanic crust is youngest near the ridge,
but it becomes progressively older away from the spreading center
due to divergence of the plates over time. This age progression
is demonstrated in the image below.

Age of the Atlantic oceanic crust.
The crust near the continental margins (blue) is about 200 million
years old. It gets progressively younger toward the mid-Atlantic
ridge, where oceanic crust is forming today. Courtesy of NOAA.

Whereas oceanic crust is generated
at divergent plate margins, it is consumed at convergent plate
margins. For a more detailed description of volcanism at convergent
plate boundaries and intraplate settings, link to: